1. Field of the Invention
The present invention relates to a section of pipe for a gas treatment device in which the gas is excited by a surface-wave plasma, particularly at atmospheric pressure.
2. Description of the Related Art
A known type of exciter device effective for this application is known by the name xe2x80x9csurfaguidexe2x80x9d or surface waveguide.
This type of device comprises a hollow structure forming a waveguide made of electrically conductive material, intended to be connected to a microwave generator and provided with a passage through which a pipe made of dielectric material passes, in which the gas to be excited flows, and with a region for concentrating the microwave radiation produced by the generator onto the pipe for the purpose of producing, during operation of the device, a surface-wave plasma in the gas.
One particularly advantageous application of this type of device is the plasma treatment of a chemically non-reactive gas containing impurities consisting of perfluorinated greenhouse-effect gaseous compounds.
In particular, it allows the gas to be treated and the impurities that it contains to be placed in an electric field sufficiently intense to produce an electrical discharge by ionization of the gas molecules, this discharge being caused by electrons being stripped off the molecules of the initially neutral gas.
Under the action of the discharge, the molecules of the base gas, that is to say the gas to be purified, are dissociated in order to form radicals smaller in size than the initial molecules and, consequently, where appropriate, individual atoms, these atoms and fragments of molecules thus excited giving rise to substantially no chemical reaction.
After passing through the discharge, the atoms and molecules of the base gas undergo de-excitation and recombination, respectively, in order to re-emerge intact.
On the other hand, the impurities, by being excited, undergo irreversible dissociation and transformation, forming new molecular fragments having chemical properties different from those of the initial molecules, which are then capable of being extracted from the gas by a suitable subsequent treatment.
This type of technique allows large volumes of gas to be effectively treated. However, its reliability is relatively poor because of the fact that the gaseous medium present in the discharge has a relatively high concentration of high-energy fluorinated radical species which make this gaseous medium particularly corrosive with respect to the wall of the pipe in which the plasma is established.
For example, when a pipe made of a silica tube having a thickness of 1 mm is used, this tube can be punctured in less than ten minutes in the region for concentrating the incident microwave radiation, the microwave power density being a maximum in this region.
Furthermore, when nitrogen is used as the plasma gas, a gas which is very difficult to ionize and generates particularly hot plasmas, the heating of the tube in the region for concentrating the radiation generally causes it to melt, even in the absence of a fluorinated compound, and this happens relatively quickly.
Finally, during operation, the silica tube is subjected to very severe thermomechanical stresses liable to result in the appearance of cracks.
The object of the invention is to overcome the aforementioned drawbacks, on the one hand by reducing the corrosion of the pipe in which the discharge is maintained and on the other hand by allowing normal operation of the device to be maintained even after the appearance of a crack in the wall in contact with the plasma.
The subject of the invention is therefore a section of pipe for a device for treating a gas by exciting the latter by means of incident microwave radiation suitable for producing a surface-wave plasma in the gas, the section of pipe including a discharge tube made of dielectric material intended to pass through the device in a region for concentrating the incident radiation, and through which the gas to be excited is intended to flow, characterized in that the tube comprises, over at least part of its length, a double wall, the walls lying in a heat-exchange relationship.
The discharge resistance of the tube is thus improved, while retaining effective removal of the heat given off by the plasma.
In various embodiments, the section of pipe according to the invention may furthermore comprise one or more of the following characteristics, taken in isolation or in any technically possible combination:
the space between the inner and outer walls of the tube is filled with a thermally conductive dielectric material which is relatively insensitive to the chemical species coming from the plasma.
Thus, even after the appearance of a crack in the inner wall of the discharge tube, the species capable of migrating through the crack are stopped by the layer of material lying between the walls of the tube.
the material filling the space between the inner and outer walls comprises boron nitride;
the inner wall of the discharge tube in a part intended to be placed in the region for concentrating the incident radiation comprises sapphire;
on either side of the part intended to be placed in the region for concentrating the incident radiation, the inner wall of the discharge tube comprises a ceramic;
the ceramic is a ceramic based on aluminium nitride or on alumina;
the inner wall of the discharge tube is made of a ceramic based on aluminium nitride;
the outer wall of the discharge tube is made of a ceramic based on aluminium nitride;
the outer wall of the discharge tube comprises silica;
the inner tube is made by stacking lengths of tube which are provided with ends of complementary shape;
it furthermore comprises a second tube made of dielectric material for jacketing a cooling fluid, in which the discharge tube is mounted, the second tube together with the outer wall of the latter defining a chamber for cooling the discharge tube, this chamber being intended to be connected to a source of dielectric heat-transfer cooling fluid;
the cooling fluid comprises an isoparaffinic polyalphaolefin;
it comprises means for mounting the cooling-fluid jacketing tube around the discharge tube in a sliding manner so as to allow these tubes to expand;
the means for mounting the tubes comprise at least one support provided with a stepped internal passage into which one of the end regions of each tube passes, the passage comprising a first end bore, in which a member for holding one end of the discharge tube and for holding a fitting which ensures that the discharge tube is supplied with gas to be treated is mounted, a central bore, through which the discharge tube passes, and a second end bore, into which the jacketing tube is fitted with interposition of a O-ring seal;
the fitting constitutes a head capping off the end of the discharge tube and comprises a thread which engages with a complementary thread made in the holding member;
it furthermore comprises a second seal interposed between the holding member and the fitting;
the support is provided with a channel for supplying the jacketing tube with cooling fluid, this channel emerging in the internal bore, and is furthermore provided with an added sealing membrane covering the support and the holding member; and
the section of pipe comprises means for stopping the holding member from rotating in the support.
The subject of the invention is also a device for exciting a gas, of the surface-waveguide type, having a hollow structure forming a waveguide made of electrically conductive material, which structure is intended to be connected to a microwave generator and is provided with a passage through which a pipe made of dielectric material passes, in which the gas to be excited flows, and with a region for concentrating the microwave radiation produced by the generator onto the pipe for the purpose of producing, during operation of the device, a surface-wave plasma in the gas, characterized in that the pipe comprises a section of pipe as defined above.